Solid electrolyte fuel cell

ABSTRACT

A solid electrolyte which consists of a mixture of cerium oxide and gadolinium oxide or said mixture with magnesium oxide added thereto, and which has an excellent oxygen-ion-conductive characteristic at a relatively low temperature as compared with conventional solid electrolytes.

United States Patent [72] lnventors Yoshiyuki Maki Fujisawa-shi; Masayuki Matsuda, l-lachioji-shi; Tetsuichi Kudo, Tokyo, all of Japan [21] Appl. No. 836,265

[22] Filed June 25, 1969 [45] Patented Sept. 21, 1971 [73] Assignee Hitachi, Ltd.

Tokyo, Japan [32] Priority June 28, 1968 [33] Japan [54] SOLID ELECTROLYTE FUEL (ELL 2 Clalms, ll Drawing Figs.

[52] US. Cl 136/86 F, l36/l53,204/l95 [51] Int. Cl HOlm 27/00, HOlm 13/00 [50] Field of Search l36/86, 153 [56] References Cited UNITED STATES PATENTS 3,346,422 lO/l967 Berger 136/153 3,489,610 1/1970 Berger et al. l36/l53 Primary Examiner-winston A. Douglas Assistant Examiner-H. A. Feeley Attorney-Craig, Antonelli and Hill ABSTRACT: A solid electrolyte which Consists ofa mixture of cerium oxide and gadolinium oxide or said mixture with mag nesium oxide added thereto, and which has an excellent ox ygen-ion-conductive characteristic at a relatively low temv perature as compared with conventional solid electrolytes.

X (MOL NUMBER OF 6212 0 PATENIEU SEPZI l97| SHEET 2 OF 8 w 0 3 w WW .7 0 m H 2 E m 6 E7 w F A Eu qv muefifimmt uRGmmw /20 TEMPERATURE [/0 T 7n] INVENTORS yaSHIYuKL MAKI, MASAYMKL MAI'SuDA ATTORNEYS PATENTED SEP2 1197i I SPECIFIC RESISMNCE' (Q cm Z IO SHEET 3 [1F 8 FIG. 3

TEMPERA TURF PM 140' TEMPERATURE [/0 T (70] INVENTORS YasHIYuKI MAKI, MASAYKKL ATSuoA 4nd TE'ISulcr-(I KLADO ATTORNEY) PATENTEU SEPZI ml SHEET 0F 8 FIG. 4

y (MaL NUMBER OF mo) INVENTORS MAsAyuKJ. MA I'Su on YosutYuKt MARI,

and TETSHLCH'L K 400 4 7 Mil/4 M 1 ATTORNEYS PATENTED ssm I91: 3.607.424

SHEET 5 OF 8 FIG. 5

SPEC/FIG RES/STANCE (12 -67" I0 I l l I l y (MOL NUMBER OF MgO INVENTORS NS/11mm. MAKI, MASAYHKL' MAI'SHOA 4, rarsurcu E M400 ATTORNEYS SOLID ELECTROLYTE FUEL CELL BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a solid electrolyte and more particularly to a solid electrolyte most suitable for use in fuel cells.

2. Description of the Prior Art As is well known, a conventional solid electrolyte consists of a solid solution mainly of zirconium oxide in which is included 3 to 20 percent by mol of calcium oxide or rare-earth oxide, such as yttrium oxide or ytterbium oxide, to stabilize the electrolyte in a cubic form as well as to impart electric conductivity to the oxygen ion in said solid solution. It is also known that a solid electrolyte is a chemically stable substance having a small electric resistance and one of which the ion transference number is closer to I has better characteristic.

It is said that a solid electrolyte particularly suitable for use in fuel cells has an electric resistance smaller than several tens ohms or a specific resistance smaller than about 409cm. Such electric resistance generally tends to be lowered as the temperature rises but for operating a fuel cell comprising the conventional solid electrolyte described above, it has been necessary to hold the fuel cell at least to a temperature as high as 900 to 1 ,000 C. Therefore, the fuel cell comprising such conventional solid electrolyte has the disadvantages as set forth below:

1. Since the cell is operated at such a high temperature as 900 to [,000" C., the materials of which the cell is constructed are required to have excellent heat-resisting property and yet further, since these materials are generally expensive, such a platinum, the cost of the cell becomes high.

2. The proportion of heat-insulating parts used for maintaining the high temperature with respect to all component parts of the cell becomes particularly large. Consequently, the volume and weight of the cell become extremely large and the energy density per unit volume or unit weight of the cell is low.

. A large amount of heat must be supplied and, therefore, the maintenance cost becomes high because the parts which participate in the electrochemical reaction in the cell must always be maintained at a high temperature.

4. A longer time is required for operating the cell as the operation temperature of the cell becomes higher.

5. Since this type of fuel cell is generally operated at a high temperature, a metal oxide readily diffuses in the solid electrolyte at a high velocity where such metal oxide is used as electrode catalyst, and such diffusion of metal oxide accelerates the deterioration of solid electrolyte, with the accompanying result that the service life of the cell is shortened.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a solid electrolyte which has an excellent oxygen-ion-conductiv'e characteristic and a small specific resistance at a comparatively low temperature as compared with conventional solid electrolytes.

It is another object of the present invention to provide an improved fuel cell by the use of the solid electrolyte of the character described above.

It is a further object of the present invention to provide an improved apparatus for measuring a partial pressure of oxygen gas.

It is a still further object of the present invention to provide an improved oxygen refiner.

According to the present invention, there is provided a fuel cell which can be operated at a comparatively low temperature in the range from about 600 to about 800 C.

The foregoing objects can be attained by employing a solid electrolyte which consists essentially of a solid solution of the general formula:

wherein x and y are me] number of GdgOg and mol number of MgO respectively represented by curve 1 in the below-mentioned FIG. 1.

Such a solid electrolyte of the present invention can be readily obtained by the ordinary firing method. Namely, the objective solid electrolyte can be obtained by a process which comprises weighing the respective raw materials individually so as to formulate a composition which satisfies the general formula given above, thoroughly mixing said raw materials, shaping the mixture into a desired shape by adding thereto a suitable binder, such as, for example, polyvinyl alcohol, firing the shaped product for several hours at 1,500 to l,700 C and cooling the same to room temperature.

The raw materials used for preparing the solid electrolyte are not necessarily restricted to oxides only but any compounds which can be readily converted into oxides on firing, such as, for example, oxalates and carbonates, may be used.

Other objects, advantages and features of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings which show the preferred examples of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the composition of the solid electrolyte according to the present invention;

FIGS. 2, 3 and 6 are specific resistance-temperature characteristic curves illustrating different examples of the present invention respectively;

FIGS. 4 and 5 are characteristic curves showing the specific resistance of the solid electrolyte with respect to the amount of MgO added to said solid electrolyte respectively;

FIG. 7 is a sectional view showing the construction of the essential portion of a fuel cell in which the solid electrolyte of the present invention is used;

FIG. 8 is a terminal voltage-current density characteristic curve of the fuel cell shown in FIG. 7;

FIG. 9 is a sectional view of a detector in an apparatus for measuring a partial pressure of oxygen gas to which the present invention is applied;

FIG. 10 is a sectional view of an oxygen refiner in which the solid electrolyte according to the present invention is used; and

FIG. 11 is a characteristic curve of the oxygen refiner, showing the relationship between the effluent rate of produce oxygen and the voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, the characteristic curves show the relationship between the composition of the electrolyte of this invention, consisting essentially of 2 mols of CeO x mol of Gd203 and y mol of MgO, and the specific resistance of the electrolyte at specific temperatures.

The curve I is curve on which the electrolyte shows a specific resistance of 800cm. at 725 C., and the region inside of the curve is a region in which the electrolyte shows a specific resistance smaller than 809cm. and the region outside of the curve is a region in which the electrolyte shows a specific resistance larger than 800cm.

The curve 2 is a curve on which the electrolyte shows a specific resistance of 300cm. at 800 C. and the region inside of this curve is a region in which the electrolyte shows a specific resistance smaller than 300cm. Namely, this curve represents an electrolyte which is superior to that represented by the curve 1.

The curve 3 is a curve which represents a particularly preferable electrolyte according to the present invention and said electrolyte has a specific resistance of 300cm. at 723 C. The region inside of the curve is one in which the electrolyte has excellent characteristics, such as a specific resistance smaller than 300cm. at said temperature.

The sample electrolytes used for obtaining the characteristic curves shown in FIG. 1 were prepared by the following process. Namely, as raw materials, fixed amount, that is, 2

mols, of CeO was mixed with varying amounts of cap, and MgO, and the mixture was thoroughly mixed in a ball mill for several hours, while grinding the same. Thereafter, the mixture was subjected to a preliminary firing in air at 1,200 to l,300 C. for 8 hours and the thus-fired mixture was again ground and mixed thoroughly in a ball mill for several hours. After adding polyvinyl alcohol as binder, the powder thus obtained was pressure-molded into a disc shape and the shaped product was subjected to a final firing in air at l,600 C. for 15 hours.

According to the present invention, it is possible to obtain a solid electrolyte having a specific resistance smaller than 800cm. at 725 C., by selecting the values of x and y in the region inside of the curve 1 in FIG. 1; a solid electrolyte having a specific resistance smaller than 309 cm. at 800 C., by selecting the values of x and y in the region inside of the curve 2; and a particularly preferable solid electrolyte having a specific resistance smaller than 309cm. at 723 C., by selecting the values of x and y in the region inside of the curve 3.

Upon examining the crystal structure through X-ray diffraction, it was revealed that the solid electrolyte according to the present invention constitutes a solid solution. It was also confirmed that the solid electrolyte of the present invention permits only oxygen gas to permeate therethrough and its transference number of oxygen ion is at least 0.95 or greater and is substantially close to l. In other words, the solid electrolyte of the present invention has excellent oxygen-ion-conductive characteristic and its electron conductivity is negligibly low.

Referring now to FIG. 2, there is shown the relationship between the temperature and the specific resistance, i.e. the specific resistance-temperature characteristic, of solid electrolytes according to the present invention, in comparison with that of a conventional solid electrolyte. A curve I represents the characteristic of the conventional solid electrolyte (the best one of the conventional solid electrolytes) for the purpose of comparison, and curves 2 and 3 respectively represent the characteristics of different solid electrolytes according to the present invention. The sample solid electrolytes of the present invention used for obtaining the curves 2 and 3 were prepared in the same manner as those of FIG. 1 and their compositions are shown in table I together with the composition of the conventional solid electrolyte used for comparison.

TABLE 1 Curve No. Chemical composition 1 :)o.oo( a)o.m

Conventional solid electrolyte As may be apparent from FIG. 2, there is a remarkable difference between the conventional solid electrolyte and the present ones. Comparing both solid electrolytes with each other with respect to specific resistance, for example, at 700 C., while the specific resistance of the conventional solid electrolyte, represented by the curve 1, is about 1 100cm, that of the present solid electrolyte, represented by the curve 2, is about 130cm. and that of the present solid electrolyte, represented by the curve 3, about 400cm. This clearly shows how excellent the solid electrolytes of the present invention are.

The solid electrolyte of the present invention, the transference number of oxygen ion of which is 0.95 or greater at 600 to 800 C., is well serviceable for use as a solid electrolyte in said temperature range.

The solid electrolyte of the present invention having such an excellent specific resistance-temperature characteristic as describe above, when used in a fuel cell, brings about a remarkable advantage, i.e. the fuel cell can be operated at a temperature as low as 600 to 800 C. which is 200 to 300 C. lower than than the operation temperature of the conventional fuel cell. Therefore, it is possible not only to reduce the production cost of the fuel cell drastically relative to the conventional one, but also to reduce the maintenance cost of the cell since the cell is operable at a relatively low temperature.

The solid electrolyte of this invention can be highly advantageously used, not only in a fuel cell as described above but also in various apparatus, e.g. an oxygen refiner and an apparatus for measuring a partial pressure of oxygen gas, in which an electrochemical reaction is applied.

Example I CeO Gd O and MgO were mixed at the mol ratio of 2:0.4:0.l and the mixture was pressure-molded into a disc shape, with polyvinyl alcohol added thereto as binder. The shaped product was fired in air at l,600 C. for 15 hours, whereby an objective solid electrolyte was obtained. The sample solid electrolyte thus obtained showed a specific resistance-temperature characteristic as represented by the curve 2 in FIG. 2. The transference number of oxygen ion of the sample solid electrolyte was substantially close to l and the specific resistance thereof was also very low and was 130cm. for example, at 700 C.

Example 2 Examples 3 to 8 Solid electrolytes were produced in the same manner as in example 1 but by mixing CeO,, 04,0, and MgO in the proportions shown in table 2 below. These solid electrolytes had specific resistance--temperature characteristics as shown in FIG. 3 respectively.

TABLE 2 Curve Transference No. in number of Example No. FIG. 3 Chemical composition oxygen ion 4 1):( s)o.1o( 8 )o.5o i0. 95

The curve 1 which is shown in FIG. 3 for comparison, represents the characteristic of a conventional solid electrolyte consisting of (ZrO Y OQ It will be clear from the above examples that the specific resistances of the solid electrolytes in the respective examples are all smaller than that of the conventional one at 650 to 900 C., which signifies that the present solid electrolytes have excellent characteristics.

Example 9 Sample solid electrolytes were produced in exactly the same manner as in example 1 by mixing CeO, 0e 0, and MgO at the moi ratio of 2:0.4:y and the specific resistances of the respective samples were measured at 723 C. to obtain a curve as shown in FIG. 4, which shows the relationship between the specific resistance and the mol number y of MgO. The transference number of oxygen ion of the solid electrolyte was 0.97 or greater at any value ofy in FIG. 4. As may be clear from FIG. 4, the effect of adding MgO appears from the amount of MgO of about 0.25 mol of smaller and is particularly apparent when the amount of MgO is from 0.07 to 0.17, namely a specific resistance of as low as 9.5 to 109cm. can be obtained.

Example Sample solid electrolytes were produced in exactly the same manner as in example 9 above, by mixing CeO Gd O and MgO at the mol ration of 220.6:y, and the specific resistances of the respective samples were measured at 720 C. to obtain a characteristic curve as shown in FIG. 5, which shows the relationship between the specific resistance and the mol number y of MgO. The transference number of oxygen ion of the solid electrolyte was 0.95 or greater at any value ofy in FIG. 5. As may be clear from FIG. 5, in the composition an effective result can be obtained when y O.2 but the best result in the vicinity ofy=0 (the specific resistance being 289cm), namely no effect of adding MgO can be recognized.

Examples l l to 17 By mixing the respective raw materials at the mol ratios shown in table 3, sample solid electrolytes were produced in exactly the same manner as in example I and the specific resistance-temperature characteristics of these samples were examined, with a result as shown in FIG. 6. In FIG. 6, a curve 1 represents the characteristic of a conventional solid electrolyte of composition (ZrO Y O which is shown for comparison.

TABLE 3 Curve Transference No. in number of Example No. FIG. 3 Chemical composition oxygen ion 10 (CeO,);(Gd,O )0.,5 g0. 95

Example 18 FIG. 7 is a vertical sectional view showing the essential portion of a fuel cell of oxygen-hydrogen type in which the solid electrolyte of this invention is used. A container 71 consists of a heat-resistant tube, and a solid electrolyte 72 according to this invention is molded in the shape of a tube having a wall thickness of 0.1 cm. and composed of (CeO (Gd O MgO) Disposed along the outer and inner peripheral surfaces of the tubular solid electrolyte 72 are a positive electrode 73 and a negative electrode 741 respectively each of which consists, for example, of a porous silver electrode comprising a silver screen and a metal oxide layer formed thereon. Reference numeral 75 designates heat-supporting means for maintaining the essential portion of the cell at a high temperature and reference character R designates a load. The fuel cell having a construction as described above was operated by supplying hydrogen gas into the tubular solid electrolyte 72 as fuel and oxygen gas or air into the annular space defined by the outer wall of said tubular solid electrolyte 72 and the inner wall of the heat-resistant tube 71, while maintaining said respective parts at 723 C. by the heat-supporting means 75, whereby a terminal voltage-current density curve as shown in FIG. 8 was obtained. In FIG. 8, a curve 17 represents the characteristic curve of the fuel cell using the solid electrolyte of this invention and a curve 18 represents the characteristic curve of a fuel cell using a conventional solid electrolyte composed of (ZrO Y O shown for comparison. As may be clear from FIG. 8, in the prior art fuel cell the terminal voltage drops abnormally as the discharge current density is increased, whereas in the fuel cell according to the present invention the terminal voltage drops only slightly. The activation polarizations of these electrolytes are both on the order of 0.05v. and a major portion of the terminal voltage drop is on account of ohm drop (i.e. IR drop). The terminal voltage drop of the fuel cell comprising the present solid electrolyte is very small because the specific resistance of the solid electrolyte is small as compared with the conventional one.

Namely, the fuel cell incorporating the solid electrolyte of this invention is advantageous, not only in that it is operable at a lower temperature than the fuel cell incorporating the conventional electrolyte as mentioned previously but also in that it operates with a very small energy loss, owing to its low specific resistance.

The solid electrolyte according to the present invention is highly excellent over the conventional one in respect of specific resistance-temperature characteristic and is highly stable chemically, as stated above. Therefore, it will be obvious that application of the present solid electrolyte is not necessarily restricted only to fuel cells but the solid electrolyte can be used in the other apparatus, such as an oxygen refiner and an apparatus for measuring a partial pressure of oxygen gas as mentioned before.

FIG. 9 is a vertical sectional view showing briefly the construction ofa detector in an apparatus for measuring a partial pressure of oxygen gas, exemplifying another application of the present solid electrolyte. In FIG. 9, reference numeral 91 designates the solid electrolyte according to the present invention which is molded in the shape of a crucible, 92 and 92 electrodes of the detector provided on the outer and inner wall surfaces of the solid electrolyte, 93 a supporting tube for the solid electrolyte which is made of a heat-resistant material such as alumina, nd 94 and 94 conductors of the detector having one ends thereof connected with said electrodes 92 and 92' respectively. When the detector constructed as described above is placed in an atmosphere of unknown partial pressure of oxygen gas, with a space 95, defined by the electrolyte 91 and the supporting tube 93, being filled with a reference gas of a known oxygen gas partial pressure, an electromotive force is developed across the electrodes 92 and 92' through the electrolyte 91, according to a partial pressure differential of oxygen gas between both sides of said electrolyte. By measuring such electromotive force through the conductors 94 and 94, it is possible to know the partial pressure of oxygen gas in the atmosphere.

The use of the above-described detector incorporating the solid electrolyte of this invention is advantageous in obtaining an apparatus for measuring a partial pressure of oxygen gas which is electrically highly stable and operates with high accuracy, because the electric resistance of the solid electrolyte proper is smaller than that of the conventional electrolyte.

FIG. I0 exemplifies the application of the present solid electrolyte to an oxygen refiner. A solid electrolyte disc 21 used in the oxygen refiner was prepared by mixing 2 mols of CeO 0.4 mol of Gd O and 0.1 mol of MgO, moldingthe mixture with pressure at room temperature to form a disc of 1 mm. in thickness and 10 mm. in diameter, firing the disc at l,200 to l,300 C. in the atmosphere for 8 hours, after cooling regrinding the disc again into a powder, molding the powder into a disc and then firing the disc at l,600 C. for 15 hours. The disc 21 thus produced was then placed between a porous silver cathode electrode 22. and a porous silver anode electrode 23 and assembled therewith. The assembly was sealed in a cylindrical heat-resistant tube or container 24 of such a material as alumina, zirconia or magnesia, in such a manner as to divide the interior of said sealed tube into separate chambers 2.2a and 23a. An electric potential of up to 20 volts was imposed across the two electrodes 22. and 23 through conductors 29 and 3t). On the other hand, the whole cell was held at 700 C. by heatsupporting means (not shown) and an oxygen-containing gas was introduced into the chamber 22a through a conduit 25. A conduit 26 permits egress of an exhaust gas. The effluent gas stream emerging from the porous silver electrode 23 into the chamber 23a was pure oxygen.

Referring to P16. 11 there is shown a characteristic curve representing the relationship between the electric potential imposed across the two electrodes 22 and 23 through the conductors 29 and 30, and the effluent rate of the pure oxygen gas emerging through said electrodes. In FIG. 11, a curve 19 represents the oxygen refiner incorporating the solid electrolyte (CeO,),(Gd,0,) (MgO),, according to the present invention and a curve 20 represents one incorporating a conventional solid electrolyte consisting of 85 mol percent of ZrO, and mol percent of CaO.

It will be seen from a comparison between these two curves that the apparatus using the present solid electrolyte has an excellent performance and this is because of the fact that the electric resistance of the present solid electrolyte is incomparably smaller than that of the conventional solid electrolyte. In practice, the specific resistance of the present solid electrolyte at 700C. is about 139cm, whereas that of the conventional solid electrolyte is about 100flcm.

The solid electrolytes of the invention suitable for use in the aforesaid fuel cell, detector for oxygen gas and oxygen refiner, etc. are selected from those whose compositions fall in the region inside of the curve 2, preferably in the region inside of the curve 3, in FIG. 1.

What is claimed is:

l. A fuel cell of oxygen-hydrogen type, comprising (a) negative electrode plates, (b) positive electrode plates, (c) a solid electrolyte, (d) a container and (e) heat-,supportin g means, said solid electrolyte consisting essentially of a solid solution represented by the general formula 2)2( 2 s).r( 8 wherein at and y are mol numbers of 011,0, and MgO respectively which are selected within a region surrounded by a curve 2 shown in HQ 1.

2. A fuel cell of oxygen-hydrogen type as defined in claim 1, in which x and y representing mol numbers of Gd O and MgO respectively are selected within a region surrounded by a curve 3 shown in FIG. 1.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,607,424 Dated September 21, 1971 Invento -(s) et a].

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The Japanese priority application No, "48/44601" should read Signed and sealed this 2nd day of May 1972.

Attest:

EDWARD M.FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents OHM PC1-1050 (H169) 

2. A fuel cell of oxygen-hydrogen type as defined in claim 1, in which x and y representing mol numbers of Gd2O3 and MgO respectively are selected within a region surrounded by a curve 3 shown in FIG.
 1. 